Dremel.sup.1 type rotary power tools have found considerable commercial success for several decades because of their low-cost and capability of accepting a wide variety of drills, grinding wheels, milling tools, and many other tool bits. One typical Dremel tool manufactured by Dremel is the "Multi-Pro.TM..sup.2, Model 395, Type 5. This tool is a variable speed tool having a speed range from 5,000 to 30,000 rpm. The tool body is hand-held, elongated, and has a spindle coaxial with the central axis of the drive motor. The spindle is selectively locked in position by a radial push button that permits the user to screw and unscrew a collet nut on the spindle to gain access to the collet for removal or replacement. FNT 1. Dremel is a registered trademark of S. B. Power Tool Co. of Chicago, Ill. FNT 2. Multi-Pro is a trademark of S. B. Power Tool Co. of Chicago, Ill.
The collets designed for this particular tool are one-piece collets, and each collet is capable of accepting a single size tool shank; i.e., there is a separate collet for each shank size, 1/32nd, 1/16th, 3/32nd, and 1/8th inches. The collets themselves are one-piece sleeves having straight uniform diameter through bores there-through with small slots extending axially along the collets that enable the collet jaws to move radially a small distance. Because the through bore is of uniform diameter and the slots are very small, each collet is capable of accepting only a single diameter tool shank.
Each collet has an enlarged head portion that forms the jaws of the collet with tapered rear and forward outer surfaces that are cammed respectively by the end of the spindle and the interior frusto-conical surface of the collet nut.
To change collets, the user rotates the output spindle to the appropriate position to be locked and unscrews the collet nut from the spindle and removes the seated collet by grasping the collet head, replaces that collet with another size, and threads the collet nut back on the spindle, then inserts a tool shank having about the same diameter as the collet bore and continues screwing the collet nut down with the spindle lock button depressed.
While the Dremel type tool is an excellent tool for the hobbyist and small industrial jobs, the requirement for multiple collets is a considerable cost addition to the product and it necessitates frequent collet changing by the ultimate user, so it is somewhat of a nuisance to operate.
Thus, it would be desirable if the Dremel type tool could be redesigned in some fashion to eliminate the need for frequent collet replacement.
The co-pending application, U.S. Ser. No. 09/129,153, referred to above, attempts to ameliorate the collet change problem in Dremel type tools, but does so at the expense of complicating the tool design.
In that application, a tool bit holder is provided for use with a tool having a rotatable output shaft with a proximal end and a distal end, the output shaft having a hollow portion of predetermined diameter adjacent its distal end, and having a threaded portion, with the tool holder comprising a collet nut having a threaded portion for threaded engagement with the threaded portion of the tool output shaft, and having a hollow interior at least a portion of which is of greater diameter than the predetermined diameter of the hollow portion of the output shaft, and having an interiorly disposed camming surface, and having a collet disposed in said hollow interior portion of the collet nut and having a plurality of radially disposed resilient jaws each having a head portion and a shank portion, with the head portions having a first camming surface for camming engagement with the interiorly disposed camming surface of the collet nut, and the head portions having a second camming surface, and with the shank portions having a common free end which is receivable in the output shaft hollow portion and an annular expansion insert member disposed in said hollow interior of the collet nut and having an aperture of predetermined diameter through which the shank portions of the collet pass, the expansion insert member having a proximal end for bearing against the distal end of the tool output shaft, and having a distal end for bearing against the second camming surfaces of the head portions of the collet nut.
While this design works well and may be acceptable for many applications, it does somewhat add to the cost of the product because of the additional insert and a second disadvantage results from the requirement that the collet assembly including the collet nut extends further from the tool housing by a distance equal to the axial length of the annular insert. The problem with this is that you have excessive tool overhang that creates tool operation instability, increases tool run out, and reduces operator control.
Therefore, it would be desirable to eliminate the need for this insert, but thus far such a redesign has not been foreseeable prior to the present invention.
There are also a plurality of prior patents that disclose collet nut and spindle assemblies having one-piece collets. The following patents are included in that list:
______________________________________ United States Patents Inventor Patent No. Issue Date Goodell 141,345 July 29, 1873 Trump Nov. 14, 1876 Whitehead Apr. 18, 1911 Parker June 25, 1918 Jones Jan. 28, 1930 Corley May 3, 19312 Oster Aug. 13, 1940 Benjamin Jan. 14, 1941 Cohen Feb. 15, 1944 Benjamin, et al. 2,358,300 Sept. 19, 1944 Godfrey Apr. 24, 1945 Mogilnicki Jan. 24, 1995 ______________________________________